The present invention relates to a piston structure for an engine, in which the piston is coupled to a smaller end part of a connecting rod by a piston pin.
Generally, in engines installed in vehicles (e.g., automobiles), a piston is coupled to a smaller end part of a connecting rod by a piston pin. Specifically, the piston pin is inserted through a pin insertion hole formed in the smaller end part of the connecting rod, and the smaller end part of the connecting rod is located in a central area of the piston pin in its axial directions. Two boss parts are formed in a bottom face of the piston (the face opposite to a top face), at positions corresponding to both end portions of the piston pin in the axial directions so as to sandwich the smaller end part of the connecting rod therebetween. The two boss parts are formed with pin supporting holes into which both the end portions of the piston pin in the axial directions are inserted and for supporting both the end portions, respectively (e.g., see JP2004-353500A).
Such engines having the above configuration are known to cause combustion noises due to resonance caused depending on a basic structure of the engine (e.g., see Masaya Otsuka, “How to Minimize Diesel Combustion Noise by Improving Engine Structure,” Proceedings of Society of Automotive Engineers Convention, No. 36-05, Society of Automotive Engineers of Japan, Inc. May 2005, P. 7-10). In “How to Minimize Diesel Combustion Noise by Improving Engine Structure,” it is described that the engine sound has three peaks at 1.7 kHz, 3.3 kHz, and 6 kHz. One of these peaks (3.3 kHz) is caused by stretching resonance of the connecting rod, and the amplitude of this resonance can hardly be reduced.
Note that the present invention has a related earlier application (JP2012-189134).
The present inventors have studied for spring mass models for pistons and connecting rods, and as a result, they have found out the followings.
In spring mass models for pistons and connecting rods, a piston, a piston pin, and a smaller end part of a connecting rod correspond to a mass point (wherein the mass is M (unit: kg)) as a whole, and a coupling part of the connecting rod coupling the smaller end part and a larger end part thereof corresponds to a spring (wherein the spring constant is K (unit: N/m)) supporting the mass point at the larger end part. Thus, if the piston, the piston pin, and the smaller end part of the connecting rod act integrally, these components integrally resonate with respect to the larger end part of the connecting rod at a resonance frequency of (½π)×(K/M)1/2 Hz (e.g., 3 kHz-4 kHz). This resonance corresponds to the stretching resonance of the connecting rod described above.
Meanwhile, a lubricant film is formed between the piston pin and the pin insertion hole of the connecting rod. The lubricant film corresponds to the spring coupling the piston pin to the smaller end part of the connecting rod. Moreover, in a case where a full floating type assembly, in which the piston pin is turnable with respect to all the boss parts and the smaller end part of the connecting rod, is adopted, in addition to between the piston pin and the pin insertion hole of the connecting rod, lubricant films are also formed between the piston pin and each of the pin supporting holes of the boss parts of the piston. The lubricant films correspond to the springs coupling the piston pin to the piston.
With the lubricant film between the piston pin and the pin insertion hole of the connecting rod (in the full floating type, this lubricant film and the lubricant films between the piston pin and the pin supporting holes of the boss parts of the piston), the piston is supported by the smaller end part of the connecting rod via the spring, and the piston, the piston pin, and the smaller end part of the connecting rod do not integrally resonate with respect to the larger end part of the connecting rod. Since the piston is not pushed with a strong force except for on combustion stroke (expansion stroke), the lubricant film remains, and thereby, the resonance does not occur.
On the other hand, on the combustion stroke, since the piston is pushed with a strong force, the lubricant film is eliminated, and as a result, the piston, the piston pin, and the smaller end part of the connecting rod integrally resonate with respect to the larger end part of the connecting rod.
From these point of views, since the piston, the piston pin, and the smaller end part of the connecting rod become integral on the combustion stroke, it can be considered to utilize a dynamic absorber in order to suppress the resonance (reduce vibration at the resonance frequency). However, by simply providing the dynamic absorber, although the noises due to the resonance can be reduced on the combustion stroke, the noises increase due to the vibration of the dynamic absorber on other strokes where the piston, the piston pin, and the smaller end part of the connecting rod do not become integral.